1. Field of the Invention
The present invention relates to a vehicle steering control apparatus which can automatically drive a vehicle along a road on the basis of both road end position information (e.g. leftward and rightward white lines) and in response to steering direction instructions.
2. Description of the Prior Art
An example of prior-art vehicle steering control apparatus will be explained with reference to FIG. 1. In FIG. 1, two cameras 7 and 8 mounted on a front portion of a vehicle 4 is switched from the leftward camera 7 to the rightward camera 8 or vice versa in accordance with a predetermined travel plan, in order to detect a road end position data such as a distance X.sub.L between a leftward white line 5 and a vehicle longitudinal axis y at a frontward observation point OP (a distance L ahead of the vehicle 4) and a rightward road end position data such as a distance X.sub.R between a rightward white line 6 and the vehicle longitudinal axis y, or a leftward inclination angle .phi..sub.L between the vehicle axis y and the white line 5 or 6 and a rightward inclination angle .phi..sub.R between the vehicle axis y and the white line 5 or 6. The steering operation of the vehicle 4 is self-controlled on the basis of these detected steering data. In more detail, in the case where the vehicle travel plan is previously determined in such a way that the vehicle is first turned to the left and then to the right, the leftward camera 7 mounted on the vehicle front portion detects the leftward data X.sub.L or .phi..sub.L at the left turn and the leftward steering angle .phi..sub.L is determined on the basis of the detected leftward data. Further, the rightward camera 8 detects the rightward data X.sub.R or .phi..sub.R at the succeeding right turn and the rightward steering angle .theta..sub.R is determined on the basis of the detected rightward data. Therefore, the steering angle .theta..sub.out to be outputted is simply determined as the leftward steering angle .theta..sub.L at the left turn and as the rightward steering angle .theta..sub.R at the right turn.
Here, the leftward camera 7 is switched to the rightward camera 8, for instance, at the timing when the leftward steering inclination angle .phi..sub.L detected as the leftward road end position (white line) data is reduced below a predetermined value (zero or almost zero) after a steering direction instruction (for switching the steering angle output .theta..sub.out to the rightward steering angle .theta..sub.R) has been inputted to the control system. This is because when the vehicle 4 has been steered almost in parallel to the leftward white line 5 after the left turn, it is possible to consider that the left turn has been completed.
In the prior-art steering control apparatus, however, when the vehicle is turned consecutively from the left to the right or vice versa, since the camera (i.e. steering direction) switching timing is determined on the basis of only the inclination angle, there exists a problem in that the steering angle is not switched smoothly at the same predetermined value at the left and right turns, so that the steering operation is not effected smoothly and continuously, thus causing steering vibration.
In particular, where the vehicle runs along a crank-shaped road with a short straight line section; that is, where the vehicle is quickly turned to the left and then right or vice versa, the camera switching timing is too late for the succeeding steering operation, even if the steering angle is switched from the left steering angle to the right steering angle or vice versa, after the inclination angle detected as the white line data has been reduced down to almost zero or the vehicle has been driven in parallel to the leftward white line, so that the vehicle is not sufficiently turned to the left. To overcome this problem, when the steering angle is switched from the left steering angle to the right steering angle before the vehicle is sufficiently steered in parallel to the leftward white line (e.g. when the inclination angle is reduced below 3 degrees), although the vehicle can be sufficiently turned to the right, since the steering angle obtained on the basis of the leftward white line immediately before the steering is switched is not continuous to that obtained on the basis of the rightward white line after the steering has been switched, thus also causing steering vibration.